Surtseyan explosions, an ash plume, and floating pumice rafts during August 2021

Fukutoku-Oka-no-ba is a submarine volcano located 5 km NE of the Japanese island of Minami-Ioto, about 1,300 km S of Tokyo. Typical activity at the site has consisted of discolored water and floating pumice. An island (called “Shin-Ioto”) was present during 1904-1905 and 1914. The 1986 eruption also built an island 600 m long and 15 m high which was later eroded away, according to information from the Japan Coast Guard. An explosion above the water surface was observed on 3 February 2010, which continued into that April (BGVN 41:06). Intermittent observations of discolored water have been made since that time, but there has been no notable eruptive activity until strong explosions in mid-August 2021 sent ash plumes to altitudes of over 12 km. Observations and monthly and special information reports come from the Japan Meteorological Agency (JMA), Japan Coast Guard (JCG) and Japan Maritime Self Defense Force (JMSDF), as well as various satellite data.

Summary of activity during May 2014-July 2021. Between May 2014 and 6 March 2016, the Japan Coast Guard intermittently visited the site, though no reports of water discoloration were made (table 4). Beginning on 7 March 2016, a pale green discoloration was noted in the water, measuring 200 x 1,000 m. Similar variations of greenish discolored water continued intermittently leading up to the eruption on 13 August 2021.

Table 4. Summary of observations made at Fukutoku-Oka-no-Ba between October 2015 and December 2021 by Japanese authorities. The Japan Coast Guard continued to visit the site intermittently, noting occasional water discoloration. Sources: JMA = Monthly Summary Report, Japan Meteorological Agency, JMSDF = Japan Maritime Self Defense Force, TRCGH = Third Regional Coast Guard Headquarters, JCG = Japan Coast Guard.

Visit Date	Discolored Water	Dimensions	Description	Source
11 Jun 2014	No	--	--	MSDF
13 Jun 2014	No	--	--	TRCGH
25 Jun 2014	No	--	--	JCG
12 Sep 2014	No	--	--	MSDF
26 Nov 2014	No	--	--	TRCGH
26 Dec 2014	No	--	--	MSDF
27 Jan 2015	No	--	--	TRCGH
15 Feb 2015	No	--	--	TRCGH
18 Feb 2015	No	--	--	TRCGH
14 Apr 2015	No	--	--	TRCGH
05 May 2015	No	--	--	TRCGH
26 May 2015	No	--	--	TRCGH
17 Aug 2015	No	--	--	JCG
23 Aug 2015	No	--	--	TRCGH
18 Oct 2015	No	--	--	TRCGH
07 Mar 2016	Yes	200 x 1,000 m	Very pale green	TRCGH
29 Mar 2016	No	--	--	JCG
24 Dec 2016	No	--	--	JCG
09 Feb 2017	No	--	--	MSDF
07 Mar 2017	Yes	500 m radius	Light green, four locations vent	JMSDF
24 Mar 2017	Yes	50 x 1,000 m NE	Yellow-green	JCG
30 Mar 2017	Yes	700 m diameter	Pale green	MSDF
24 Apr 2017	No	--	--	MSDF
28 Jun 2017	No	--	--	JCG
03 Jul 2017	No	--	--	MSDF
06 Oct 2017	No	--	--	MSDF
11 Dec 2017	Yes	2 x 6 km	Pale green	JCG
27 Jan 2018	Yes	600 m diameter	Yellow-green	MSDF
24 Feb 2018	Yes	500 x 1,000 m	Light green	MSDF
27 Apr 2018	No	--	--	MSDF
18 Jul 2018	No	--	--	JCG
29 Jan 2019	Yes	1 km diameter	Light green	JCG
19 Nov 2019	No	--	--	JCG
04 Feb 2020	Yes	--	Whitish yellow-green drifting E	JCG
15 Mar 2020	Yes	--	Bluish-white	JCG
16 Apr 2020	Yes	--	Yellow-green	TRCGH
19 Apr 2020	Yes	--	Bluish white drifting E	TRCGH
18 May 2020	No	--	--	TRCGH
19 Jun 2020	No	--	--	JCG
05 Apr 2021	Yes	--	Pale blue	JCG
08 Jun 2021	Yes	2.4 km diameter N	Bluish-white	JCG
09 Jul 2021	Yes	--	Bluish-white	JCG
05 Aug 2021	Yes	--	Bluish-white drifted SSW	JCG
13 Aug 2021	--	--	Eruption began, though observations could not be made	TRCGH
15 Aug 2021	--	1 km diameter	New island with intermittent eruptions and floating pumice drifting 60 km NW	TRCGH
16 Aug 2021	Yes	--	Gas-and-steam plume from the island, suspended pumice drifted 100 km WNW	JCG
26 Aug 2021	Yes	--	Brown drifting W, suspended pumice	JCG
12 Sep 2021	Yes	2 km diameter	Yellowish-green to yellowish-brown, second yellow-green discolored area 2 km ENE of the island	JCG
12 Oct 2021	Yes	--	Brown and yellow-green drifting 5.6 km NW, white waves	JCG
01 Nov 2021	Yes	--	Bluish-white	TRCGH
11 Nov 2021	Yes	3 km diameter and 2 km diameter	Dark yellow-green and a second yellow-green with floating pumice	JCG
29 Nov 2021	Yes	--	Brown to yellowish-green	TRCGH
14 Dec 2021	Yes	--	Yellow-green and bluish-white	TRCGH
27 Dec 2021	Yes	--	Brown and yellowish-green	JCG

Observers on four overflights during 2020 reported visible discolored water originating from a single location, which has been frequently noted since 2005, according to JMA. On 4 February 2020 the Japan Coast Guard (JCG) conducted an overflight that confirmed pale white- to yellow-green discolored water on the ocean surface near the volcano over a distance of about 6.3 km to the W (figure 16). Discolored bluish-white water was again reported by JCG during an overflight on 15 March, though it was fainter than the previous observations. Similar faint discolored water was reported on 16 and 19 April by JCG. No further activity was reported for the rest of 2020. Similar activity was observed in overflights by JCG during the first half of 2021. Faintly discolored water (light blue to white) was observed during overflights on 5 April, 8 June, and 9 July (figure 17) according to JCG.

Eruption during August 2021. During aerial observations on 5 August the JCG only saw a blue-white discolored area SSW of the volcano. A large eruption starting at 0620 on 13 August 2021 was reported by JCG, based on satellite data, and was later confirmed during an overflight that day. The resulting ash plume rose as high as 16 km altitude and had drifted 700-1,000 km W by 0900 (figure 18). Video data showed Surtseyan explosions that ejected black material above the ocean surface and drifted W. Ash plumes identified in satellite images rose 12.2-16 km altitude during 13-15 August (figure 19) and eventually reached the N part of Luzon Island in the Philippines (2,000 km SW); the height decreased after the initial ejection, though activity continued. During an overflight on 15 August the JCG observed a new island (also referred to as “Niijima”) that measured about 1 km in diameter, where gas-and-steam emissions were detected rising from its center on 16 August (figure 20). Brown discoloration of a variable shape, but with a consistent diameter of 1 km, was visible surrounding the new island. Additionally, a notable floating pumice raft was photographed extending up to 60-100 km WNW and was about 13 km wide (figure 21). A Sentinel-2 natural color satellite image captured the newly formed island, surrounded by yellow-green discolored water that extended E the next day on 17 August (figure 22).

A local fisherman about 5 km SSW near South Iwo Jima (Minami-Ioto) posted photos from 17 (figure 23), 20, and 22 August showing a plume rising above the volcano. On 20 August lightning was visible within the plume. Another plume was observed during 0430-0630 on 22 August and some of the floating pumice was collected by the Ocean Meteorological Observation vessel, Keikaze Maru (figure 24). The largest sample was about 40 cm in diameter; samples were mainly white, gray, and dark gray with visible bubbles, which JMA reported was similar to the samples collected from the 1986 eruption. During an overflight on 26 August observers noted that while the W part of the island remained unchanged, the E side had mostly eroded, yielding two small islands (figure 25). Though no eruptive activity was visible, gray material from the central vent was intermittently ejected to the ocean surface, and brown discolored water remained widely distributed, extending especially to the W.

Activity during September-December 2021. JMA reported that the yellow-green to yellow-brown discolored water continued to be widely distributed in the area during September. By 12 September an overflight showed that the E side of the island had been completely eroded and submerged (figure 26), while the length of the W side measured 1 km. Yellow-green to yellow-brown discolored water extended from the vent area to the SW, S, and SE. In addition, on 12 September an area of yellow-green to yellow-brown discolored water 2 km in diameter, independent of the initial eruption site, was observed 2 km ENE of the new island (figure 27). Floating pumice were also noted around the discolored area. As a result, JMA issued a navigation warning to nearby vessels.

Discolored water areas persisted into October, as well as floating pumice. By early October the JCG reported that the floating pumice that was ejected during the eruption in mid-August had reached the coastlines more than 1,000 km from the volcano. Pumice first reached Minamidaitou Island (1,000 km W) on 4 October (figure 28) and samples were collected and compared to those collected on 22 August. Pumice was later reported at Kitadaito Island (1,000 km W) during 5-8 October. Aerial observations reported that the W side of the new island had eroded to a third of the size it was when previously described on 12 September (figure 29). Brownish discolored water was distributed near the N shore of the W part of the island, accompanied by yellow-green discoloration, extending about 6 km NW of the volcano (figure 29). Later in the month, the pumice circled Okinawa and Maejima islands (1,400 km W) and several ports in the Kagoshima prefecture (1,300 km NW), which damaged hundreds of boats and ships, clogged harbors (figure 30), and impacted the fishing and tourism industries in several areas, which led local governments to begin removing the pumice from the water.

During an overflight on 11 November white fumarolic plumes were observed rising from the N end of the island, accompanied by a blue-white band of discolored water in the S part of the island and a dark yellow-green band on the W side (figure 31). Circular discoloration was also observed on the N side of the island, along with bubbling that was seen on the ocean surface near the same location. On 27 December observers reported that the island that formed in mid-August had become smaller since 14 December and had almost eroded below the ocean surface. No eruptive activity was observed, though discolored brownish water was visible from the E end of the island. Yellow-green water and a 400-m-long string of floating pumice was circulating 5 km E of the volcano. Discolored water was visible surrounding almost the entire coast of Minami-Ioto (5 km SSW).

During December the brown-yellow discoloration continued to spread around the island, which by 27 December had noticeably decreased in size since seen on 29 November. Blue-white discoloration expanded to the NNE. Brown discolored water was visible from the E side of the island as yellow-green discoloration was distributed 5 km E of the same location. On 27 December the island was covered with white waves and was surrounded by blue-green discolored water (figure 32).

Satellite data. Landsat-8 satellite images were used to track the occurrence and direction of the discolored water plumes originating at the volcano (figure 33). Discoloration was detected during February 2021 and then reappeared after the start of the eruption in August. Accompanying the discolored water is the emergence of the new island that formed as a result of the eruption, which visibly decreased in size from August to October. A strong sulfur dioxide plume was captured in the Sentinel-5P satellite on 13 August 2021, drifting W as far as 350 km and rising to 3 km altitude (figure 34). The plume exceeded 18 DUs (Dobson Units).

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Japan Coast Guard (JCG) Volcano Database, Hydrographic and Oceanographic Department, 3-1-1, Kasumigaseki, Chiyoda-ku, Tokyo 100-8932, Japan (URL: http://www.kaiho.mlit.go.jp/info/kouhou/h29/index.html); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard MD 20771, USA (URL: https://so2.gsfc.nasa.gov/); NHK Japan Broadcasting Corporation, 2-2-1 Jinnan, Shibuya-ku, Tokyo 150-8001, Japan (URL: https://www.nhk.or.jp/, https://twitter.com/nhk_news/media); Yukio Hayakawa, Gunma University, Japan, 371-8510 Gunma, Maebashi, Aramakimachi, 4-2 (URL: http://www.hayakawayukio.jp/, https://twitter.com/HayakawaYukio); Yutaka Kosugi (URL: https://twitter.com/yutaka_kosugi); German Aerospace Center (DLR), Linder Höhe, 51147 Koeln, Germany (URL: https://www.dlr.de/en); Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Ringlaan 3, Avenue Circulaire 3, 1180 Brussels (URL: https://www.aeronomie.be/index.php/en); The European Space Agency (ESA), 24 rue du Général Bertrand, CS 30798, 75345 Paris CEDEX 7, France (URL: https://www.esa.int/).

Information is preliminary and subject to change. All times are local (unless otherwise noted)

March 1975 (CSLP 24-75)

Eruption building new island

Card 2130 (28 March 1975) Eruption building new island

The following cable was received from Fleet Weather Central, Guam. "On 25 March 1975 a U.S. Air Force reconnaissance flight operating out of Guam sighted an area of oceanic volcanic activity 32 km S of Iwo Jima. The sighting was described as possibly an island in the formative stages. Photographs were taken by the crew. Fleet Weather Central, Guam, is closely watching weather satellite coverage for the area for any signs of further activity."

Information Contacts: Fleet Weather Central, U.S. Air Force, Guam, Mariana Islands.

January 1977 (NSEB 02:01) Cite this Report

Possible submarine eruption in December 1974

[A table of possible submarine eruptions based on aerial observations of water discoloration by the Japanese Maritime Safety Agency included an entry for 24.28°N, 141.50°E, in December 1974. This location is very close to Fukutoku-Okanoba. In March 1975 another observation was listed at 24.42°N, 141.32°E; this is ~25 km NW of Fukutoku-Okanoba and ~36 km S of Iwo-jima.]

Information Contacts: AFP; U.S. Defense Mapping Agency.

April 1977 (NSEB 02:04) Cite this Report

Water discoloration observed during August 1976

[A report of surface discoloration in March was removed at JMA's request.] The last reported activity in this area was a [green] discoloration in August 1976.

Information Contacts: JMA, Tokyo.

August 1977 (NSEB 02:08) Cite this Report

Sea surface discoloration in May

Sea surface discoloration was observed from the air on 1, [12, 13, 16, 17, 21, 22, and 25 May].

Information Contacts: JMA, Tokyo; D. Shackelford, CA.

May 1978 (SEAN 03:05) Cite this Report

Water discoloration observed October-December 1977 and January 1978

Discoloration was observed on 14 October 1977, 15 and 25 November 1977, 8 and 23 December 1977, and 10, 24, and 25 January 1978.

Information Contacts: JMA, Tokyo.

March 1979 (SEAN 04:03) Cite this Report

Sea water discoloration

Aerial reconnaissance by the JMSA in late January showed slight discoloration of the ocean around this volcano, at 24.30°N, 141.48°E.

Information Contacts: Y. Sawada, Meteorolgical Research Institute, Tokyo; D. Shackelford, CA.

July 1979 (SEAN 04:07) Cite this Report

Discolored water observed in 1978 and 1979

Activity has been observed at six submarine volcanoes in the Volcano Island chain in 1978 and 1979 (figure 1). Discolored water has often been visible . . . during frequent overflights by JMSDF and JMSA aircraft. [Discolored water was noted, in an included table of observations, by almost every overflight during January 1978-July 1979 (see table 1 below).]

Information Contacts: JMSA, Tokyo; JMA, Tokyo.

April 1980 (SEAN 05:04) Cite this Report

Discolored water in late 1979 and early 1980

The Japan Maritime Safety Agency continues frequent monitoring flights over three known submarine volcanoes. [Discolored water at Fukutoku-Okanoba was seen during September 1979-March 1980 (see table 1 below).]

Information Contacts: JMSA, Tokyo; JMA, Tokyo.

November 1980 (SEAN 05:11) Cite this Report

Discolored water seen during April-July 1980

The Japan Maritime Safety Agency (JMSA) continues frequent monitoring flights over several known submarine volcanoes. [Discolored water at Fukutoku-Okanoba was seen during April-July 1980 (see table 1 below).]

Information Contacts: JMSA, Tokyo; JMA, Tokyo.

December 1980 (SEAN 05:12) Cite this Report

Water discoloration observed in November and December 1980

The Japanese Maritime Safety Agency (JMSA) continues frequent aerial monitoring of several known submarine volcanoes. [Discolored water was seen at Fukutoku-Okanoba on 14 November and 18 December 1980 (see table 1 below).]

Information Contacts: JMSA, Tokyo; JMA, Tokyo.

October 1981 (SEAN 06:10) Cite this Report

Frequent water discoloration observed during January-October 1981

The Japan Maritime Safety Agency (JMSA) continues frequent monitoring flights over several known submarine volcanoes. [Discolored water at Fukutoku-Okanoba was specifically noted, in an included table of observations, on 9 and 29 January, 12 and 20 February, 12, 13, and 16 March, 20 April, 16, 17, and 22 July, 17 August, and 17 September 1981 (see table 1 below).]

Information Contacts: JMSA, Tokyo; JMA, Tokyo.

September 1982 (SEAN 07:09) Cite this Report

Discolored water observed on flights during December 1981-June 1982

JMSA [and JMSDF have] continued frequent monitoring flights over several known submarine volcanoes (table 1). On almost all overflights discolored sea water was seen around Fukutoku-Okanoba, the most active submarine volcano in this area.

Table 1. Volcanic activity at three sites (Fukutoku-Okanoba, Minami-Hiyoshi, and Fukujin) in the Volcano Islands, January 1978-December 1982, detected by JMSA and JMSDF monitoring flights. Activity indicated as D = Discolored water; N = No discolored water, - = No overflight, ? = not reported. This table combines data from reports in 1979 (4:7), 1980 (5:4, 5:11, and 5:12), 1981 (6:10), and 1982 (7:9, plus additional data not published in the Bulletin).

Date	Fukutoku-Okanoba	Minami-Hiyoshi	Fukujin
10 Jan 1978	D	N	D
24 Jan 1978	D	N	D
25 Jan 1978	D	-	D
26 Jan 1978	-	D	D
23 Feb 1978	D	D	N
24 Mar 1978	D	D	D
13 Apr 1978	D	N	N
25 May 1978	D	N	N
28 Jun 1978	D	-	-
29 Jun 1978	D	N	N
25 Aug 1978	D	N	D
24 Oct 1978	N	N	N
15 Nov 1978	D	-	-
16 Nov 1978	D	N	N
14 Dec 1978	D	N	N
11 Jan 1979	D	-	-
24 Jan 1979	D	N	N
08 Feb 1979	D	N	N
27 Mar 1979	D	[N]	-
26 Apr 1979	D	N	D
15 Jun 1979	N	N	N
11 Jul 1979	N	-	-
12 Jul 1979	D	N	D
24 Aug 1979	N	N	-
09 Sep 1979	D	-	-
13 Sep 1979	D	N	N
25 Oct 1979	D	N	D
08 Nov 1979	D	N	D
12 Dec 1979	D	N	D
29 Jan 1980	-	-	D
15 Feb 1980	D	N	D
19 Mar 1980	D	N	D
24 Apr 1980	D	N	N
12 May 1980	D	-	D
16 Jun 1980	D	N	N
07 Jul 1980	D	-	-
08 Jul 1980	D	N	N
14 Jul 1980	D	N	N
18 Aug 1980	N	N	N
04 Sep 1980	N	N	N
21 Oct 1980	N	N	N
14 Nov 1980	D	N	N
18 Dec 1980	D	N	N
07 Jan 1981	-	-	D
08 Jan 1981	-	-	D
09 Jan 1981	D	N	N
29 Jan 1981	D	-	-
12 Feb 1981	D	[-]	-
20 Feb 1981	D	[N]	N
12 Mar 1981	D	[-]	-
13 Mar 1981	D	N	N
16 Mar 1981	D	N	N
20 Apr 1981	D	N	N
17 Jun 1981	N	N	N
16 Jul 1981	D	-	-
17 Jul 1981	D	[N]	[N]
22 Jul 1981	D	N	N
17 Aug 1981	D	N	N
17 Sep 1981	D	N	N
19 Oct 1981	N	N	N
11 Nov 1981	N	N	[-]
21 Dec 1981	D	N	N
12 Jan 1982	-	-	D
19 Jan 1982	D	N	D
09 Feb 1982	D	N	N
28 Feb 1982	D	-	-
16 Mar 1982	D	N	D
01 Apr 1982	D	-	N
02 Apr 1982	D	N	N
16 Apr 1982	D	N	N
14 May 1982	D	N	N
16 Jun 1982	D	N	N
22 Jun 1982	D	-	-
13 Jul 1982	D	?	N
20 Aug 1982	D	?	N
27 Sep 1982	D	?	N
13 Oct 1982	D	?	N
17 Nov 1982	D	?	N
15 Dec 1982	D	?	D

Information Contacts: JMA, Tokyo.

March 1984 (SEAN 09:03) Cite this Report

Discolored water observed on 12 days in 1983

Frequent monitoring of several known submarine volcanoes has continued. . . . [Discolored water was observed at Fukutoku-Okanoba in 1983 on 14 January, 2 and 15 February, 3 and 15-16 March, 24 May, 16 June, 20 July, 25 October, 11 November, and 12 December; and was not visible 12 July, 25 August, and 13 September.]

Information Contacts: JMA, Tokyo.

October 1985 (SEAN 10:10) Cite this Report

Discolored water

A milky white discoloration, about 100 m in diameter, was observed on 17 September and discoloration was noted again on 17 October.

Information Contacts: JMA, Tokyo.

November 1985 (SEAN 10:11) Cite this Report

Water discoloration observed frequently during December 1983-October 1985

JMSA has continued frequent aerial monitoring of several known submarine volcanoes. Volcanic activity has often been observed at Fukutoku-Okanoba during overflights since late 1983 . . . . Discolored water was observed at Fukutoku-Okanoba on: 21 December 1983; 30 January, 23 February, 15 March, 6 and 23 April, 18 May, 9 June, 10 July, 1 August, 6 September, and 16 November 1984; and 14 February, 15 March, 17 April, 14 May, 17 September, and 17 October 1985; and was not visible 26 September, 24 October, and 12 December 1984.

[Additional water discoloration in 1985 was reported (Bulletin of Volcanic Eruptions no. 25, 1988) on 14 June, 17 July, 13 August, 14 November, and 23 December.]

Information Contacts: JMA, Tokyo.

December 1985 (SEAN 10:12) Cite this Report

Explosive eruption builds island

On 18 January, fishermen observed a large white plume rising to more than 3 km altitude and lava being ejected to about 300 m height form the vicinity of the submarine volcano Fukutoku-Okanoba. The eruption was first evident on infrared imagery from the Japanese GMS geostationary weather satellite on 18 January at 2100. Japan Maritime Self-Defense Force personnel stationed on Iwo-Jima, about 50 km from Fukutoku-Okanoba, observed a 4-km eruption cloud the next day at 1630. The Japan Maritime Safety Agency's survey ship Takuyo found a new island about 5 km NE of Minami Iwo Jima (south Iwo Jima) the morning of 21 January. The new island, about 700 m long and 300 m wide, extended about 15 m above sea level. Lava was being ejected to about 300 m height. As of 21 January, floating pumice had drifted 60 km to the SE.

Information Contacts: JMA, Tokyo; UPI; AP

January 1986 (SEAN 11:01) Cite this Report

New island formed

An eruption . . . built a small island in mid-January. Only milky blue discolored sea water was observed near the volcano on 16 January at about 1200. By 18 January at 2000, eruptive activity was frequent, producing a dark plume, seen from a fishing boat, that reached 3,000-4,000 m height. An hour later the plume was evident on an image from Japan's GMS satellite. The next day at about 1630, airborne radar detected a plume rising to 4,000 m above sea level. On 20 January at 0440, radar on the JMSA's survey ship RV Takuyo showed an island-like image. A gray plume rose to 2,000 m altitude at 0550.

A new island was recognized by observers on the RV Takuyo at 0630. A 50-m vent at the NW edge of the new island ejected plumes and sea water. Continuous observations were made until about 1000. The island was ~700 m long, 500 m wide, and 15 m high (figure 2). A vent near its E edge remained below sea level, ejecting black incandescent rocks as much as 10 m in diameter. Grayish brown pumice had drifted as much as 18 km NE from the vent. Discolored sea water surrounded the island and extended 22 km NE from the vent. The sea water was greenish near the island, gradually becoming greenish yellow or pale blue farther from the vent.

At 1122 a dark plume containing incandescent rocks to 10 m in diameter rose 1,200-1,300 m above sea level; between 1250 and 1400 a vapor plume rose to 3,000 m. There was a zone of discolored water ~45 km long and 2-3 km wide. Between 1450 and 1550, explosions occurred at about 20-minute intervals, expelling dark plumes that rose ~1,200-1,700 m. Between 1600 and sunset, 90 weak to moderate explosions ejected plumes to 100-1,000 m height. Volcanic flame was occasionally seen within dark plumes at night.

The next morning at 0800 explosions produced small plumes (less than 50 m high) at intervals of 1-2 minutes. At 1223, a large explosion was seen; volcanic flame reached 1,500 m above sea level, roughly half of the plume's height. At 1300 the island was flat and crescent-shaped, covered with ash and pumice but no lava flow. The outer arc of the crescent was a portion of a circle ~700 m in diameter having a center that roughly coincided with the position of the active vent, which was ~300 m in diameter (figures 3 and 4). An area of thin yellow or green discoloration extended SSE to 2 km from the island. Pumice drifted E and S.

By the morning of the 22nd explosive activity had subsided; no plumes were seen for the rest of the month. Erosion of the shoreline of the new island was evident early the next day. However, at 1300 outflow of discolored sea water suggested that the submarine vent, 80 m in diameter, was still active ~400 m E of the island.

The island, covered with gray ash, was ~500 m in length and had a maximum width of 200 m. During the morning of 25 January, dense blue discoloration was seen around and E of the island and had drifted somewhat eastward, to ~9 km from the vent. On the 29th the island's size was estimated at ~600 m long, ~400 m wide and ~10 m high (this preliminary estimate does not necessarily indicate growth since the 23rd). Yellowish green discoloration extended as much as 38 km to the NE, and brown pumice was floating in lines near the island.

Between 20 December and 20 January, nearly 100 T-phase waves of moderate intensity were received by high-gain seismic stations at Rangiroa, Tuamoto Islands (~9,000 km SE of Fukutoku-Okanoba). J.M. Talandier noted that T-phase waves are characteristic of submarine eruptions, having shallow (ocean) depth. The origin of these waves was in the NW Pacific, and they may be related to the eruption of Fukutoku-Okanoba.

Discolored water has frequently been observed around Fukutoku-Okanoba in recent years. In an interview on a Japan Broadcasting Network newscast, Professor Kosaka of Okayama Univ reported that about 8 years ago a sonar survey from the Tokyo Fisheries Univ ship Seio-maru detected a seamount with a summit 40 m below sea level and roughly 150 m wide, beneath the discolored water zone at Fukutoku-Okanoba. Small islands were formed during the 1904-1905 and 1914 eruptions, but did not persist after 1905 and 1916. An island was reported during an eruption on 24 December 1974, but was not visible the next day.

Information Contacts: JMA; J. Talandier, Laboratoire de Geophysique, Tahiti; NHK Japan Broadcasting Corporation, Tokyo.

February 1986 (SEAN 11:02) Cite this Report

New island erodes; floating pumice

The island formed 20 January . . . has been eroded away by wave action since volcanic activity ceased. The island's disappearance was reported by the JMSDF after an hour-long helicopter flight that began 8 March at 0745. The surface of the new edifice was seen under white water, and there was no sign of a volcanic plume.

On 28 January, airplane pilots observed light brown floating pumice within a roughly rectangular NW-SE-trending zone ~200 km long by 50 km wide, extending from ~100 to 300 km SE of the volcano. The pumice was in subparallel teardrop-shaped rafts roughly 4 km wide and 10 km long, elongate perpendicular to the apparent direction of drift.

Rocks collected from Fukutoku-Okanoba after previous eruptions in 1904, 1914, and 1982 were trachyandesites with high alkali contents.

Information Contacts: M. Matson, NOAA/NESDIS; Kyodo Radio, Tokyo; UPI.

March 1986 (SEAN 11:03) Cite this Report

January island eroded below sea level

Water discoloration was seen on every overflight in February and March. Wave action gradually eroded the island built by the January eruption, and JMSA personnel reported that it was entirely below sea level by the time of their overflight on 26 March.

Information Contacts: JMA; JMSA.

June 1986 (SEAN 11:06) Cite this Report

Discolored sea water

Airplane pilots have observed discolored water at the volcano during all overflights after the temporary island was eroded below sea level in late March. No eruptive activity was reported. The length of the zone of discolored sea water was 0.3 km on 16 April, 2 km on 19 May, and 25 km on 18 June.

Information Contacts: JMA; JMSA.

July 1987 (SEAN 12:07) Cite this Report

Submarine eruptions; floating pumice

On 14 July at about 1240, discolored and spouting seawater was reported by observers on a fishing boat. Sprays reached 10 m above the water surface for about a minute at intervals of 10-15 minutes. The next day, 100-150-m-diameter, circular, white-crested waves with spouts in their centers formed at intervals of 6 minutes throughout a 50-minute JMSA overflight. Some floating pumice was found within a zone of yellowish-green seawater that was 500 m wide and stretched 18 km E. No white-crested waves formed after 16 July. During a 17 July overflight, a 30-100 m-wide area of green seawater extended 13 km E. Pumice floated at the E end of this area. On 21 July discolored seawater spread out from two points and minor amounts of pumice were floating in the area. On 30 July deep-green discolored water extended as far as 9 km E over an area 0.5-2 km wide and contained floating brown pumice.

Information Contacts: JMA.

September 1987 (SEAN 12:09) Cite this Report

Discolored and spouting seawater

Discolored seawater continued to be seen in August by the JMSA. On 1 August, a zone of yellow-green seawater stretched 3 km NW-SE. The next day water over the vent was discolored green. Discolored water was next reported on 13 August, when cobalt-blue water covered an area of 0.01 x 4 km along an E-W axis. On 17 August the discolored area was 3 km in diameter, and spouting seawater was seen twice. No floating pumice was observed.

Information Contacts: Y. Sawada, JMA; D. Shackelford, Fullerton, CA.

October 1987 (SEAN 12:10) Cite this Report

Pale green discolored water

JMSA observed pale green discolored water above the volcano on 17 September.

Information Contacts: Y. Sawada, JMA; D. Shackelford, Fullerton, CA.

October 1988 (SEAN 13:10) Cite this Report

Discolored water

JMSA observed sea discolorations on 12 August in an area of 0.09 x 0.9 km and on 15 September in a zone of 0.05 km (N-S) by 1.8 km (E-W).

Information Contacts: Y. Sawada, JMA; D. Shackelford, Fullerton, CA.

July 1989 (SEAN 14:07) Cite this Report

Thermal activity discolors sea water

Thermal activity has continued since the January 1986 eruption (11:1-3). A pale green belt of discolored sea water and white bubbles was observed during a JMSA overflight on 9 May 1989. Another monitoring flight, on 14 June, revealed a 50 x 400 m zone near the vent that was discolored cobalt blue.

Information Contacts: JMA; D. Shackelford, Fullerton, CA.

December 1990 (BGVN 15:12) Cite this Report

Frequent observations of discolored water

Aerial observations of the ocean surface above the submarine volcano revealed discolored water in January-May, September, and November (table 2). The longest discoloration, 8 km, was observed during an overflight on 17 January. Thermal activity has continued since the January 1986 eruption . . . . Discolored water has been frequently observed since that time (14:07). No discolorations were observed at other submarine volcanoes (Kaitoku, Minami-Hiyoshi, Nikko, and Fukujin seamounts) during 1990.

Table 2. Discolored water at Fukutoku-Okanoba, observed from monthly overflights in 1990. Courtesy of JMSA.

Date	Water Color	Length of Discoloration
17 Jan 1990	yellowish brown	8 km
20 Feb 1990	green	7 km
27 Mar 1990	green and brown	1.8 km
17 Apr 1990	blue	0.3 km
16 May 1990	blue	0.05 km
Jun 1990	no discoloration	--
Jul 1990	no discoloration	--
Aug 1990	no discoloration	--
13 Sep 1990	pale green	3.6 km
15 Oct 1990	no discoloration	--
20 Nov 1990	yellowish green	0.7 km
Dec 1990	no discoloration	--

Information Contacts: Hydrographic Department, JMSA.

April 1991 (BGVN 16:04) Cite this Report

Water discoloration during one of five overflights

A blue water discoloration, extending 2 km E-W, was observed during a 6 February overflight by the JMSA. Overflights on 18 January, 12 March, 15 April, and 10 May revealed no abnormal water.

Information Contacts: JMA.

September 1991 (BGVN 16:09) Cite this Report

Water discoloration over submarine vent

A greenish water discoloration was observed during a 30 September overflight by the JMSA. No discoloration was present during a similar overflight on 21 August.

Information Contacts: JMA.

October 1991 (BGVN 16:10) Cite this Report

Discolored water

An 18 October overflight by the JMSA revealed discolored green water over the submarine volcano.

Information Contacts: JMA.

October 1992 (BGVN 17:10) Cite this Report

Pumice ejected by submarine eruption

A dark plume was observed [in the air above Fukutoku-Okanoba] by fishing crews at about 0800 on 10 November. An overflight by the JMSA from 1315 to 1350 revealed discolored pale-green water in a narrow fan shape to 10 km NNW from the volcano. Brown pumice was seen but no ejections were observed at that time. An overflight on 12 November between 1215 and 1250 revealed that water discoloration was reduced to a zone extending 1 km NW without floating pumice, indicating a decrease in activity. An area of discolored water 2 km long was seen in August but no discoloration was observed during the last overflight before the eruption, on 22 October.

Information Contacts: JMA; JMSA.

November 1992 (BGVN 17:11) Cite this Report

Smaller zone of discolored water; no new pumice

Overflights by the JMSA on 12 and 18 November revealed a smaller and weaker area of discolored water over the submarine volcano than on 10 November. The height and size of the dark plume seen by fishermen on 10 November are not known, nor is is known whether the plume contained tephra. Floating pumice was observed that day, but none has been reported since then.

Information Contacts: JMA; JMSA.

December 1993 (BGVN 18:12) Cite this Report

Discolored water observed over volcano four times in 1993

Discolored seawater over an area a few hundred meters wide was seen during monthly aerial observations . . . in February, March, June, and September 1993.

Information Contacts: JMA.

December 1995 (BGVN 20:11) Cite this Report

Discolored seawater

An aviator flying over the waters of the southern Volcano Islands for Japan's Maritime Safety Agency reported seeing light-green seawater on 25, 27, and 28 November. Discolored seawater was last seen at this location in September 1993.

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

January 1996 (BGVN 21:01) Cite this Report

Discolored seawater observed for the second time in three months

On 12, 22, and 23 January, an aviator from the Japan Marine Safety Agency (JMSA) reported distinct discoloration of seawater to yellowish green. Similar discoloration was seen during 25-28 November 1995 (BGVN 20:11/12). Prior to that, discolored seawater was last seen at this location in September 1993.

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

March 1996 (BGVN 21:03) Cite this Report

Emission of colored water and long discolored aqueous plumes

Aviators from the Japan Marine Safety Agency (JMSA) began observing yellowish-green discoloration of seawater during 25-28 November 1995 (BGVN 20:11/12). Similar discoloration was seen on 12, 22, and 23 January 1996 (BGVN 21:01), and also on 26 January, as reported by the Japan Meteorological Agency.

Information from the Volcano Research Center revealed that JMSA observers once again noted yellowish brown discolored seawater in the area on 4 April. According to the reports, the colored area expanded like a belt up to ~3 km long. Strong emission of colored water was recognized from two points. Although white-colored suspension was observed on the surface, floating pumices were not recognized. Yellowish-green to yellowish-brown water observed on 12 April formed a plume ~4 km long and 200 m wide, including 3-4 spots from which colored-water was gushing out intermittently. No pumices were recognized.

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan; Volcano Research Center, Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html); Hydrographic Department, Maritime Safety Agency, 3-1 Tsukiji, 5-Chome, Chuo-ku, Tokyo 104, Japan.

April 1996 (BGVN 21:04) Cite this Report

Yellowish-brown discolored seawater seen again

On 4 April, an aviator from the Japan Marine Safety Agency reported discoloration of seawater to yellowish brown at Fukutoku-Okanoba (BGVN 21:03). On 6 and 7 April, similar discoloration was observed by an aviator of the Maritime Defense Force. More discoloration was reported on 12 and 30 April. No floating pumice was observed. Discolored seawater has been seen on a smaller scale at this location, since 24 November 1995 (BGVN 20:11/12).

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan.

May 1996 (BGVN 21:05) Cite this Report

Discolored seawater

During the first half of May, aviators of the Maritime Safety Agency and the Maritime Self-Defense Force reported discoloration of seawater at Fukutoku-Okanoba. Similar discoloration has been observed since November 1995 (BGVN 20:11/12, 21:01, 21:03, and 21:04). An overflight on 23 May indicated no discolored seawater.

Information Contacts: Japan Meteorological Agency (JMA), Volcanological Division, Seismological and Volcanological Department, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan.

October 1996 (BGVN 21:10) Cite this Report

Water discoloration from submarine volcano observed in September

On 6 September, a Japan Maritime Safety Agency aviator reported yellow-green (in part yellowish brown) discoloration of seawater at Fukutoku-Okanoba over an area of 50 x 150 m, and to light green over a 300 x 500 m area. On 18 September green discoloration was seen by an aviator of the Japan Maritime Self-Defense Force. Smaller areas of discolored seawater have been seen at this location since 24 November 1995 (BGVN 20:11/12, v. 21, nos.1 and 3-5).

Information Contacts: Volcanological Division, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan; Volcano Research Center, Earthquake Research Institute (ERI), University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html).

January 1997 (BGVN 22:01) Cite this Report

Submarine venting continues to discolor seawater

On 12 December, a Japan Maritime Safety Agency aviator reported yellow-green discoloration of seawater at Fukutoku-Okanoba over a 50 x 200 m area. On 27 January, pale green discoloration was seen by an aviator of the Japan Maritime Safety Agency covering an area of 2,000 x 100 m.

Information Contacts: Volcanological Division, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan.

November 2005 (BGVN 30:11) Cite this Report

July 2005 submarine eruption; discolored water and debris

Notice of Fukutoku-Okanoba unrest in 2005 first came to Bulletin editors from Olivier Hyvernaud, information that was amplified by the Japanese Meteorological Agency (JMA) Volcanic Activity Reports of July and October 2005. The JMA reports contain information from the Japanese Marine Defense Forces as well as the Marine Security and Safety Agency and the Tokyo Institute of Technology. In addition, a Japan Coast Guard website (see URL below) contains a more extensive (and yet untranslated) table on recent events at Fukutoku-Okanoba, which includes photos and videos of the July eruption. That table clearly illustrates activity both earlier and later than the 2-3 July eruption, and several other details not discussed here, including the observation of numerous large and steaming blocks floating on the ocean surface at mid-day on 3 July. Bulletin editors hope to decipher this table and include more details in a later report.

The last five Bulletin reports discussing or mentioning Fukutoku-Okanoba appeared in BGVN 22:01, 24:11, 24:12, 25:05, and 28:06 (1997-2003). Note that the last four cases were considered ambiguous and grouped along with reports under the heading "Acoustic signals in 1999-2000 from unknown source, Volcano Islands, Japan" and only the first case was listed under the volcano name). A 3-d view of the volcano and its setting appears as figure 5.

JMA reported that at about 1745 on 2 July 2005, a white plume was witnessed at Fukutoku-Okanoba. During an investigation at 1900 that same day, a white plume reached ~ 1 km above the sea surface. A photo taken from considerable distance was included in the JMA report, showing the plume, but the image's limited contrast has led to its exclusion here. In addition to the plume, other evidence for an eruption included debris on the sea surface. When seen on 2 July, the debris covered an area approximately 100 m wide and 300 m long.

JMA noted that 3 July aerial observations suggested that compared to the previous day, eruptive vigor and the height of the white plume had decreased. The key observation then was a zone of discolored seawater (figure 6).

JMA's report of 4 and 5 July aerial investigations noted the lack of a white vapor plume over the sea. In other words, the 2-3 July eruption had calmed, but fresh debris and seawater discoloration were still present. After that, aerial investigations on 15, 17, 20, and 21 July, again disclosed seawater discoloration, but not the presence of floating debris.

The Maritime Security and Safety Agency conducted an underwater topographical survey on 20-22 July 2005, the result of which was the discovery of two craters caused by the recent eruption. The results suggested that the topography just S of those craters was newly raised.

According to a 3 October aerial observer, the ocean surface near Fukutoku-Okanoba, then displayed a pale, blue-white discoloration, interpreted as indicative of volcanism. The area of discoloration extended ~ 300 m in length to the E and was ~ 50 m wide (N-S) at its widest point. However, in the surrounding area they saw no floating debris or plumes containing ash or steam. On 27 October, an aerial observation could not confirm the seawater discoloration.

Satellite data. M. Urai (2005) reported that three days after the 2 July 2005 eruption of Fukutoku-Okanoba, satellite remote sensing using ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) observed the discolored seawater and floating materials within 40 km of the submarine volcano. Some of this abstract follows.

"At the most dense discolored seawater area, reflectance of ASTER band 1 is 3% higher [than] the surrounding seawater. The floating materials are similar in ASTER VNIR [Very Near-Infrared Radiometer] reflectance spectra to clouds, however, the floating materials can be separated from clouds using their shape and stereo image features. The extensions of discolored seawater area and floating material detected by ASTER were 6.34 km² and 1.14 km², respectively. It is possible to estimate the scale of [a] submarine eruption using the quantitative data derived from satellite remote sensing."

Distant hydrophones. Robert Dziak and Haru Matsumoto monitor N Pacific volcano seismicity with the National Oceanic and Atmospheric Agency/Pacific Marine Environmental Laboratory (NOAA/PMEL). They initially learned of the eruption via the internet. Regarding the 2 July eruption, Dziak wrote to the Bulletin staff on 22 November 2005. Some of his messages follow.

". . . the [N] Pacific hydrophone array we use recorded seismicity during the Fukutoku-Okanoba eruption near Iwo Jima. I was aware of the eruption at the time [mid 2005] thanks to Haru [Matsumoto; he designed and built the instruments used there to record the T-wave events] forwarding a news image of the discolored water. Despite being only able to roughly locate the seismicity since it is way west of our array, I am pretty sure Fukutoku-Okanoba was the source because the arrival azimuths and timing of the signals were a match. The last earthquake activity we recorded from this area occurred on 25 September [2005] . . .. A few years ago I was contacting you [Smithsonian Institution] about our recording of harmonic tremor from a source in the Volcano Islands. The conclusion I published in JGR [Dziak and Fox, 2002] was that either Fukutoku-Okanoba or Funka-asane ([N] of Iwo Jima) was the probable source because of a history of submarine volcanic activity at both volcanoes. We have still been recording this tremor intermittently over the last few years and another pulse of it occurred during the Fukutoku-Okanoba eruption on July 2, 2005. The last occurrence was on August 22.

According to an Email from Dziak on 23 November 2005, "...I think the tremor is coming from [Fukutoku-Okanoba or Funka-asane]. I was only able to get synchronous data from the French Polynesian seismic net (Hyvernaud). They confirmed the signals but it did not help much with location because they were so far away. My thought is the source of earthquakes and tremor from these submarine volcanoes is at an ocean depth within the sound channel. This allows for very efficient seismic-acoustic coupling and acoustic propagation throughout the Pacific ocean basin."

References. Dziak, R.P., and Fox, C.G., 2002, Evidence of harmonic tremor from a submarine volcano detected across the Pacific Ocean basin: Journal of Geophysical Research, v. 107(B5), p. 2085; doi 10.1029/2001JB0001772085.

Kato, Y., 1988, Gray pumices drifted from Fukutoku-oka-no-ba to the Ryukyu Islands: Bulletin of the Volcanological Society of Japan, Second Series, v. 33, p. 21-30.

Ossaka, J., Mitsuno, C., Shibata, T., Matsuda, T., Hirabayashi, J., Tsuchide, M., Sakurai, M., and Sato, H., 1986, The 1986 submarine eruption of Fukutoku-okanoba, Part 2. Volcanic ejectas: Bull. Vol. Soc. Japan, v. 31, p. 134-135.

Urai, M., 2005, Monitoring submarine volcano with satellite remote sensing: Eos Trans, AGU, v. 86(52), Fall Meet. Suppl., Abstract 611A-1176.

Information Contacts: Olivier Hyvernaud, Laboratoire de Géophysique, BP 640 Pamatai, Tahiti, French Polynesia; Japanese Meteorological Agency (URL: http://www.jma.go.jp/JMA_HP/jma/indexe.html); Robert Dziak and Haru Matsumoto, NOAA PMEL, Hatfield Marine Science Center, 2115 SE Oregon State University Drive, Newport, OR 97365, USA; Yukio Hayakawa (URL: http://www.hayakawayukio.jp/English.html/); Daily Yomiuri News (URL: http://www.yomiuri.co.jp/); Reuters; Associated Press; Tokyo Institute of Technology, 2-12-1 O-Okayama, Meguro-ku, Tokyo 152-8551, Japan; Japan Coast Guard, Hydrographic and Oceanographic Department (URL: http://www1.kaiho.mlit.go.jp/GIJUTSUKOKUSAI/kaiikiDB/kaiyo24-2.htm); Japan Maritime Security and Safety Agency, Oceanic Information Section (URL: http://www1.kaiho.mlit.go.jp/).

January 2007 (BGVN 32:01) Cite this Report

Discolored water attributed to submarine volcanism

Oliver Hyvernaud twice notified the Bulletin of Notice to Mariners reports of discolored sea water observed on 4 October and 15 November 2006 at about 24°17'N, 141°29'E. Both warnings referred to the same general location, encompassing an area of ocean 93 km SSW of Iwo Jima. In reality, the two positions noted below are ~ 1.3 km apart, perhaps overlapping each other. The warnings were as follows:

1) Discolored water with submarine volcanic activity reported within 1,700 m of 24°17.5' N, 141°29.4 E at 0400 UTC on 4 October 2006 [ref. Notice to Mariners, 14 October 2006 (no. 41), HYDROPAC Warning 1921/06(97) North Pacific, issued 1009 UTC on 4 October 2006].

2) Discolored water with submarine volcanic activity reported within 2,000 m of 24°17.1' N, 141°28.8 E at 0236 UTC on 15 November 2006 [ref. Notice to Mariners, 2 December 2006 (no. 48), HYDROPAC Warning 2225/06(97) North Pacific, issued 1133 UTC on 16 November 2006]

For the locations given for warnings 1 and 2, the respective deviations from coincidence with Fukutoku-Okanoba (at the coordinates given above) are 1.4 and 0.8 km. That seamount frequently produces discolored seawater and is known to erupt on occasions as well, and is thus the probable source for the discolored water.

According to the Japan Meteorological Agency, observations on 15 November 2006 by the Japan Maritime Self Defense Force and aerial observations on 21 November 2006 by the Japan Coast Guard revealed areas of discolored water on the sea surface above the volcano. Those cases seemed to be caused by volcanic activities.

Information Contacts: Olivier Hyvernaud, Laboratoire de Géophysique, BP 640 Pamatai, Tahiti, French Polynesia; Notice to Mariners, National Geospatial-Intelligence Agency, National Ocean Service, and U.S. Coast Guard, Office of Corporate Relations, Public Affairs Division, MS D-54, 4600 Sangamore Road, Bethesda, MD 20816-5003, USA (URL: http://www.nga.mil/portal/site/maritime/); Japan Meteorological Agency, 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/).

February 2010 (BGVN 35:02) Cite this Report

Airborne ash and cloudy water during 3 February 2010 eruption

The submarine volcano Fukutoku-Okanoba, frequently the source of discolored water (e.g., BGVN 32:01 and 30:11), erupted on the morning of 3 February 2010, sending a plume of ash and steam 100 m into the air (figure 7) and turning the surrounding ocean a yellow-green color (figure 8). Additional observations of discolored water have been frequently documented in JMA reports since November 2006 (BGVN 32:01). 

The February 2010 eruption was discovered at about 0745 by the Japan Coast Guard patrol boat Yashima on a routine survey. The crew first noticed "smoke" coming from the surface of the ocean ~ 5 km NNE of the island of Minami-Iwo-jima. The crew also heard a blast and felt its pressure wave. The Yashima continued to monitor the site from a safe distance.

Satellite imagery provided evidence that the activity persisted for at least several days. On 9 February the underwater plume appears as a complex shape N of the volcano (figure 9). Two days later, on 11 February, the plume was spread W of the volcano (figure 10).

JMA isssued eruption alerts for 9 March and 8 April 2010. This suggests that the eruption continued into April.

Information Contacts: Japan Meteorological Agency, 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/); NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/).

June 2016 (BGVN 41:06) Cite this Report

Continued intermittent plumes of discolored water through April 2014

An explosion above the water surface from the submarine Fukutoku-Oka-no-Ba volcano was observed by the Japanese Coast Guard on 3 February 2010, and eruption alerts continued into early April of that year (BGVN 35:02). Although intermittent plumes of water discoloration after that indicated some level of ongoing activity through April 2014, the last observation of pumice on the sea surface was on 14 April 2010.

This trachyandesitic volcano is located 5 km NE of the Japanese island of Minami-Ioto, about 1,275 km S of Tokyo in the Izu-Bonin arc (figures 11 and 12). The National Catalogue of the Active Volcanoes of Japan, monthly reports by the Japan Meteorological Agency (JMA), and information tables and photographs prepared by the Japan Coast Guard (JCG) and Japan Maritime Self Defense Force (JMSDF) were the data sources for this report. A summary of the observations recorded during site visits made by Japanese authorities between April 2010 and October 2015 is given here, and additional details are provided in the table at the end of the report (table 3).

Observations from February 2010 through April 2011. On 3 February 2010, a 100-m-high ash and steam eruption emerged from the sea. Floating pumice was later observed on 14 April 2010 (referred to as "floating objects" in the translation of JMA and JCG tables). Discolored water continued to be observed around the volcano for the rest of 2010. Activity from multiple vents created blue-white, milky-white, and greenish-brown plumes of discolored seawater a few hundred meters wide that drifted as far as 2,700 m. In January 2011, blue-white and creamy-colored water plumes from multiple vents drifted NE in a fan shape approximately 500 m across (figure 13). Discolored water plumes were also observed on 8 February and 12 April 2011.

Observations from January 2012 through August 2012. Inspections of the volcano in May, July, August, and November 2011 revealed no discolored water plumes or floating objects at the site. A 150-m-diameter plume was visible again on 27 January 2012. Observations on 9 March and 19 April 2012 confirmed the continued presence of discolored water. On 11 May 2012, a 50 x 150 m milky-green-colored area was photographed by the JCG (figure 14). Additional observations of discolored water plumes were made on 13 June, 15 June, and 9 August 2012; no visits were made during September-November 2012.

Observations between December 2012 and April 2014. Green water was originating from multiple locations within a 1,000-m-diameter plume on 11 December 2012. Milky-white, bluish-white, and yellow-brown discoloration of seawater from multiple sources was again observed on 22 January 2013 (figure 15).

Plumes were observed twice in March (8 and 26), April (10 and 21), and on 12 May of 2013 but not during visits in June and July. They were seen again on 21 August and 27 September. No visits were made in October and November, but a 1,850-m-long plume drifting E was observed on 16 December 2013 by JMSDF. The last confirmed observations of discolored water plumes were on 29 January and 28 April 2014 by the JMSDF and JCG. They continued to visit the site nearly every month until October 2015, with no further observations of discolored water plumes. No further reports were made after October.

Table 3. Summary of observations made at Fukutoku-Oka-no-Ba between March 2010 and April 2014 by Japanese authorities. The Japan Coast Guard continued to visit the site almost every month through October 2015 with no further reports of water discoloration, and it was not mentioned after that date in monthly JMA reports. Sources: JMA*= National Catalogue of the Active Volcanoes in Japan, JMA= Monthly Summary Report, Japan Meteorological Agency, JMSDF= Japan Maritime Self Defense Force, TRCGH= Third Regional Coast Guard Headquarters, JCG=Japan Coast Guard.

Visit Date	Discolored Water	Dimensions	Description	Source
Mar 2010	Yes	--	--	JMA*
14 Apr 2010	Yes	360 x 2700 m, SW	Blue-white, milky-white and pale green, multiple vents, floating objects observed	JMA*/JMSDF
26 Apr 2010	Yes	--	Fan-like, multiple vents, blue and milky white, no floating objects observed	JMA*/TRCGH
16 May 2010	Yes	100 m radius	Milky, multiple vents, no floating objects observed	JMA*/TRCGH
04 Jun 2010	No	--	--	TRCGH
17 Jun 2010	Yes	100 x 200 m, EW	Blue-white, and green, multiple vents	JMA*/JMSDF
Jul 2010	Yes	--	--	JMA*
17 Sep 2010	No	--	--	JMSDF
18 Sep 2010	No	--	--	JCG
29 Sep 2010	Yes	100 x 500 m NE	Light blue, thin milky	JMA*/JCG
Oct 2010	Yes	--	--	JMA/JCG
Nov 2010	Yes	--	--	JMA/JCG/JMSDF
18 Dec 2010	Yes	280 x 1,800 m	Blue-white and green, multiple active vents, drifting S	JMA/JMSDF
19 Jan 2011	Yes	500 m NE	Multiple vents, Blue-white, milky white, greenish brown, fan shape	JMA*/JCG
24 Jan 2011	Yes	--	--	JMSDF
28 Jan 2011	Yes	--	Creamy-white	JMSDF
08 Feb 2011	Yes	50 x 2000 m NE	Multiple vents, blue-white, green	JMA*/JCG
12 Apr 2011	Yes	100 m radius	Milky-white	JMA*/JMSDF
May 2011	No	--	--	JMA*
Jul 2011	No	--	--	JMA*
06 Aug 2011	No	--	--	TRCGH
16-17 Nov 2011	No	--	--	JMA*/JCG
27 Jan 2012	Yes	150 m diameter	Milky-white, yellow-green	JMA*/TRCGH
28 Jan 2012	Yes	100 m diameter	Milky-white, yellow-green, thin blue	TRCGH
30 Jan 2012	Yes	60 x 300 m N	Light green, milky	TRCGH
09 Mar 2012	Yes	50 x 200 m	Dark milky white, pale blue	JMA*/JCG
19 Apr 2012	Yes	30 m diameter	Milky	JMA*/TRCGH
11 May 2012	Yes	50 x 150 m; 1500 m SW	Milky; thin blue	JMA*/TRCGH
16 May 2012	Yes	--	Slight thin white	TRCGH
13 Jun 2012	Yes	--	Slight pale blue-white	TRCGH
15 Jun 2012	Yes	50 m	Pale blue-white	JMA*/JMSDF
09 Aug 2012	Yes	200 x 300 m	Multiple vents, blue-white	JMA/JMSDF
23 Aug 2012	No	--	Poor visibility	JCG
11 Dec 2012	Yes	1000 m diameter	Multiple vents, thin green	JMSDF
22 Jan 2013	Yes	100 x 300 m	Multiple vents, milky-white, blue-white and light green-brown	TRCGH/JMSDF
08 Mar 2013	Yes	1000 m [500 m] diameter	Milky white and blue	JMSDF [JMA]
26 Mar 2013	Yes	500 x 1500 m	Milky white, pale blue and pale green	JCG
10 Apr 2013	Yes	630 m radius	Multiple vents, green	JMA/JMSDF
21 Apr 2013	Yes	50 m radius	Milky-white	JMA/JCG
12 May 2013	Yes	50 x 400 m S	Milky-white	JMA/JCG
25 Jun 2013	No	--	--	JMA/JMSDF
29 Jun 2013	No	--	--	JMA/JCG
17 Jul 2013	No	--	--	JMA/JCG
21 Aug 2013	Yes	--	Faint discoloration	JMA/JMSDF
24 Aug 2013	No	--	--	JMA/JCG
27 Sep 2013	Yes	450 m radius	Green water and white bubbles	JMA/JMSDF
16 Dec 2013	Yes	1850 m E	Multiple vents, green	JMA/JMSDF
28 Dec 2013	No	--	--	JMA/JCG
13 Jan 2014	No	--	--	JMA/JCG
29 Jan 2014	Yes	--	--	JMA/JMSDF
02 Feb 2014	No	--	--	JMA/JCG
28 Apr 2014	Yes	--	--	JMA/JCG

Reference. Japan Meteorological Agency, 2013, National Catalogue of the Active Volcanoes in Japan (Fourth Edition, English Version), Tokyo, Japan (http://www.data.jma.go.jp/svd/vois/data/tokyo/STOCK/souran_eng/menu.htm).

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/); Japan Coast Guard (JCG) Volcano Database, Hydrographic and Oceanographic Department, 3-1-1, Kasumigaseki, Chiyoda-ku, Tokyo 100-8932, Japan (URL: http://www1.kaiho.mlit.go.jp/GIJUTSUKOKUSAI/kaiikiDB/kaiyo24-2.htm).

November 2021 (BGVN 46:11) Cite this Report

Surtseyan explosions, an ash plume, and floating pumice rafts during August 2021

Fukutoku-Oka-no-ba is a submarine volcano located 5 km NE of the Japanese island of Minami-Ioto, about 1,300 km S of Tokyo. Typical activity at the site has consisted of discolored water and floating pumice. An island (called “Shin-Ioto”) was present during 1904-1905 and 1914. The 1986 eruption also built an island 600 m long and 15 m high which was later eroded away, according to information from the Japan Coast Guard. An explosion above the water surface was observed on 3 February 2010, which continued into that April (BGVN 41:06). Intermittent observations of discolored water have been made since that time, but there has been no notable eruptive activity until strong explosions in mid-August 2021 sent ash plumes to altitudes of over 12 km. Observations and monthly and special information reports come from the Japan Meteorological Agency (JMA), Japan Coast Guard (JCG) and Japan Maritime Self Defense Force (JMSDF), as well as various satellite data.

Summary of activity during May 2014-July 2021. Between May 2014 and 6 March 2016, the Japan Coast Guard intermittently visited the site, though no reports of water discoloration were made (table 4). Beginning on 7 March 2016, a pale green discoloration was noted in the water, measuring 200 x 1,000 m. Similar variations of greenish discolored water continued intermittently leading up to the eruption on 13 August 2021.

Table 4. Summary of observations made at Fukutoku-Oka-no-Ba between October 2015 and December 2021 by Japanese authorities. The Japan Coast Guard continued to visit the site intermittently, noting occasional water discoloration. Sources: JMA = Monthly Summary Report, Japan Meteorological Agency, JMSDF = Japan Maritime Self Defense Force, TRCGH = Third Regional Coast Guard Headquarters, JCG = Japan Coast Guard.

Visit Date	Discolored Water	Dimensions	Description	Source
11 Jun 2014	No	--	--	MSDF
13 Jun 2014	No	--	--	TRCGH
25 Jun 2014	No	--	--	JCG
12 Sep 2014	No	--	--	MSDF
26 Nov 2014	No	--	--	TRCGH
26 Dec 2014	No	--	--	MSDF
27 Jan 2015	No	--	--	TRCGH
15 Feb 2015	No	--	--	TRCGH
18 Feb 2015	No	--	--	TRCGH
14 Apr 2015	No	--	--	TRCGH
05 May 2015	No	--	--	TRCGH
26 May 2015	No	--	--	TRCGH
17 Aug 2015	No	--	--	JCG
23 Aug 2015	No	--	--	TRCGH
18 Oct 2015	No	--	--	TRCGH
07 Mar 2016	Yes	200 x 1,000 m	Very pale green	TRCGH
29 Mar 2016	No	--	--	JCG
24 Dec 2016	No	--	--	JCG
09 Feb 2017	No	--	--	MSDF
07 Mar 2017	Yes	500 m radius	Light green, four locations vent	JMSDF
24 Mar 2017	Yes	50 x 1,000 m NE	Yellow-green	JCG
30 Mar 2017	Yes	700 m diameter	Pale green	MSDF
24 Apr 2017	No	--	--	MSDF
28 Jun 2017	No	--	--	JCG
03 Jul 2017	No	--	--	MSDF
06 Oct 2017	No	--	--	MSDF
11 Dec 2017	Yes	2 x 6 km	Pale green	JCG
27 Jan 2018	Yes	600 m diameter	Yellow-green	MSDF
24 Feb 2018	Yes	500 x 1,000 m	Light green	MSDF
27 Apr 2018	No	--	--	MSDF
18 Jul 2018	No	--	--	JCG
29 Jan 2019	Yes	1 km diameter	Light green	JCG
19 Nov 2019	No	--	--	JCG
04 Feb 2020	Yes	--	Whitish yellow-green drifting E	JCG
15 Mar 2020	Yes	--	Bluish-white	JCG
16 Apr 2020	Yes	--	Yellow-green	TRCGH
19 Apr 2020	Yes	--	Bluish white drifting E	TRCGH
18 May 2020	No	--	--	TRCGH
19 Jun 2020	No	--	--	JCG
05 Apr 2021	Yes	--	Pale blue	JCG
08 Jun 2021	Yes	2.4 km diameter N	Bluish-white	JCG
09 Jul 2021	Yes	--	Bluish-white	JCG
05 Aug 2021	Yes	--	Bluish-white drifted SSW	JCG
13 Aug 2021	--	--	Eruption began, though observations could not be made	TRCGH
15 Aug 2021	--	1 km diameter	New island with intermittent eruptions and floating pumice drifting 60 km NW	TRCGH
16 Aug 2021	Yes	--	Gas-and-steam plume from the island, suspended pumice drifted 100 km WNW	JCG
26 Aug 2021	Yes	--	Brown drifting W, suspended pumice	JCG
12 Sep 2021	Yes	2 km diameter	Yellowish-green to yellowish-brown, second yellow-green discolored area 2 km ENE of the island	JCG
12 Oct 2021	Yes	--	Brown and yellow-green drifting 5.6 km NW, white waves	JCG
01 Nov 2021	Yes	--	Bluish-white	TRCGH
11 Nov 2021	Yes	3 km diameter and 2 km diameter	Dark yellow-green and a second yellow-green with floating pumice	JCG
29 Nov 2021	Yes	--	Brown to yellowish-green	TRCGH
14 Dec 2021	Yes	--	Yellow-green and bluish-white	TRCGH
27 Dec 2021	Yes	--	Brown and yellowish-green	JCG

Observers on four overflights during 2020 reported visible discolored water originating from a single location, which has been frequently noted since 2005, according to JMA. On 4 February 2020 the Japan Coast Guard (JCG) conducted an overflight that confirmed pale white- to yellow-green discolored water on the ocean surface near the volcano over a distance of about 6.3 km to the W (figure 16). Discolored bluish-white water was again reported by JCG during an overflight on 15 March, though it was fainter than the previous observations. Similar faint discolored water was reported on 16 and 19 April by JCG. No further activity was reported for the rest of 2020. Similar activity was observed in overflights by JCG during the first half of 2021. Faintly discolored water (light blue to white) was observed during overflights on 5 April, 8 June, and 9 July (figure 17) according to JCG.

Eruption during August 2021. During aerial observations on 5 August the JCG only saw a blue-white discolored area SSW of the volcano. A large eruption starting at 0620 on 13 August 2021 was reported by JCG, based on satellite data, and was later confirmed during an overflight that day. The resulting ash plume rose as high as 16 km altitude and had drifted 700-1,000 km W by 0900 (figure 18). Video data showed Surtseyan explosions that ejected black material above the ocean surface and drifted W. Ash plumes identified in satellite images rose 12.2-16 km altitude during 13-15 August (figure 19) and eventually reached the N part of Luzon Island in the Philippines (2,000 km SW); the height decreased after the initial ejection, though activity continued. During an overflight on 15 August the JCG observed a new island (also referred to as “Niijima”) that measured about 1 km in diameter, where gas-and-steam emissions were detected rising from its center on 16 August (figure 20). Brown discoloration of a variable shape, but with a consistent diameter of 1 km, was visible surrounding the new island. Additionally, a notable floating pumice raft was photographed extending up to 60-100 km WNW and was about 13 km wide (figure 21). A Sentinel-2 natural color satellite image captured the newly formed island, surrounded by yellow-green discolored water that extended E the next day on 17 August (figure 22).

A local fisherman about 5 km SSW near South Iwo Jima (Minami-Ioto) posted photos from 17 (figure 23), 20, and 22 August showing a plume rising above the volcano. On 20 August lightning was visible within the plume. Another plume was observed during 0430-0630 on 22 August and some of the floating pumice was collected by the Ocean Meteorological Observation vessel, Keikaze Maru (figure 24). The largest sample was about 40 cm in diameter; samples were mainly white, gray, and dark gray with visible bubbles, which JMA reported was similar to the samples collected from the 1986 eruption. During an overflight on 26 August observers noted that while the W part of the island remained unchanged, the E side had mostly eroded, yielding two small islands (figure 25). Though no eruptive activity was visible, gray material from the central vent was intermittently ejected to the ocean surface, and brown discolored water remained widely distributed, extending especially to the W.

Activity during September-December 2021. JMA reported that the yellow-green to yellow-brown discolored water continued to be widely distributed in the area during September. By 12 September an overflight showed that the E side of the island had been completely eroded and submerged (figure 26), while the length of the W side measured 1 km. Yellow-green to yellow-brown discolored water extended from the vent area to the SW, S, and SE. In addition, on 12 September an area of yellow-green to yellow-brown discolored water 2 km in diameter, independent of the initial eruption site, was observed 2 km ENE of the new island (figure 27). Floating pumice were also noted around the discolored area. As a result, JMA issued a navigation warning to nearby vessels.

Discolored water areas persisted into October, as well as floating pumice. By early October the JCG reported that the floating pumice that was ejected during the eruption in mid-August had reached the coastlines more than 1,000 km from the volcano. Pumice first reached Minamidaitou Island (1,000 km W) on 4 October (figure 28) and samples were collected and compared to those collected on 22 August. Pumice was later reported at Kitadaito Island (1,000 km W) during 5-8 October. Aerial observations reported that the W side of the new island had eroded to a third of the size it was when previously described on 12 September (figure 29). Brownish discolored water was distributed near the N shore of the W part of the island, accompanied by yellow-green discoloration, extending about 6 km NW of the volcano (figure 29). Later in the month, the pumice circled Okinawa and Maejima islands (1,400 km W) and several ports in the Kagoshima prefecture (1,300 km NW), which damaged hundreds of boats and ships, clogged harbors (figure 30), and impacted the fishing and tourism industries in several areas, which led local governments to begin removing the pumice from the water.

During an overflight on 11 November white fumarolic plumes were observed rising from the N end of the island, accompanied by a blue-white band of discolored water in the S part of the island and a dark yellow-green band on the W side (figure 31). Circular discoloration was also observed on the N side of the island, along with bubbling that was seen on the ocean surface near the same location. On 27 December observers reported that the island that formed in mid-August had become smaller since 14 December and had almost eroded below the ocean surface. No eruptive activity was observed, though discolored brownish water was visible from the E end of the island. Yellow-green water and a 400-m-long string of floating pumice was circulating 5 km E of the volcano. Discolored water was visible surrounding almost the entire coast of Minami-Ioto (5 km SSW).

During December the brown-yellow discoloration continued to spread around the island, which by 27 December had noticeably decreased in size since seen on 29 November. Blue-white discoloration expanded to the NNE. Brown discolored water was visible from the E side of the island as yellow-green discoloration was distributed 5 km E of the same location. On 27 December the island was covered with white waves and was surrounded by blue-green discolored water (figure 32).

Satellite data. Landsat-8 satellite images were used to track the occurrence and direction of the discolored water plumes originating at the volcano (figure 33). Discoloration was detected during February 2021 and then reappeared after the start of the eruption in August. Accompanying the discolored water is the emergence of the new island that formed as a result of the eruption, which visibly decreased in size from August to October. A strong sulfur dioxide plume was captured in the Sentinel-5P satellite on 13 August 2021, drifting W as far as 350 km and rising to 3 km altitude (figure 34). The plume exceeded 18 DUs (Dobson Units).

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Japan Coast Guard (JCG) Volcano Database, Hydrographic and Oceanographic Department, 3-1-1, Kasumigaseki, Chiyoda-ku, Tokyo 100-8932, Japan (URL: http://www.kaiho.mlit.go.jp/info/kouhou/h29/index.html); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard MD 20771, USA (URL: https://so2.gsfc.nasa.gov/); NHK Japan Broadcasting Corporation, 2-2-1 Jinnan, Shibuya-ku, Tokyo 150-8001, Japan (URL: https://www.nhk.or.jp/, https://twitter.com/nhk_news/media); Yukio Hayakawa, Gunma University, Japan, 371-8510 Gunma, Maebashi, Aramakimachi, 4-2 (URL: http://www.hayakawayukio.jp/, https://twitter.com/HayakawaYukio); Yutaka Kosugi (URL: https://twitter.com/yutaka_kosugi); German Aerospace Center (DLR), Linder Höhe, 51147 Koeln, Germany (URL: https://www.dlr.de/en); Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Ringlaan 3, Avenue Circulaire 3, 1180 Brussels (URL: https://www.aeronomie.be/index.php/en); The European Space Agency (ESA), 24 rue du Général Bertrand, CS 30798, 75345 Paris CEDEX 7, France (URL: https://www.esa.int/).